The present invention generally relates to automatic fine tuning (AFT) systems for television receivers and, more particularly, concerns an adaptive AFT system for use in a television receiver having a frequency synthesis tuner.
A conventional frequency synthesis tuner operates according to the expression: EQU fLo/N2.sup.p =fref/M=Fref;
where 2.sup.P represents the prescaler, N is a variable channel index which is related to the selected channel number and M is a fixed constant. Typically, fref=16 MHz and M=2.sup.13, and therefore Fref=1953.125 Hz.
Automatic fine tuning (AFT) is normally performed by incrementing or decrementing channel index N in response to a zero-crossing of the AFT response. The step resolution in the AFT mode is therefore fixed at 2.sup.P Fref, or 125 KHz. The maximum resolution with which the local oscillator can be tuned to the desired local oscillator frequency fLo is therefore 1/2 (125 KHz) or 62.5 KHz. This fixed resolution limits the accuracy with which a received signal may be recovered as well as the speed of carrier detection. For example, at a fixed step resolution of 125 KHz, the recovered signal may suffer undesired frequency ripple which could degrade the performance of certain signal processing circuits such as ghost cancellers, due to the use in such circuits of small tap coefficients for effecting spectral equalization. Also, this fixed step resolution adversely effects auto search circuits by requiring excessive time for carrier detection.
It is therefore an object of the present invention to overcome the foregoing deficiencies by providing a frequency synthesis tuning system in which the AFT step resolution is adaptively variable, with the step resolution being reduced from a maximum value to a minimum value as the frequency of the local oscillator fLo approaches the nominal frequency for tuning the received carrier. Tuner operation may thus be effected in a rapid and accurate manner by using a large step resolution when fLo is relatively far from the nominal frequency and using smaller values of step resolution as fLo approaches the nominal frequency. In a preferred embodiment the adaptive step resolution size is initially controlled by the AFT response and subsequently as fLo approaches the nominal frequency by the output of a ghost cancellation reference (GCR) correllator.